A known example of organic light-emitting elements, so-called organic electroluminescent elements, is an element that has a transparent electrode serving as a positive electrode, a hole-transporting layer, a light-emitting layer (organic light-emitting layer), an electron-injection layer, and an electrode serving as a negative electrode stacked in this order on a surface of a transparent substrate. When voltage is applied between the positive and negative electrodes, the electrons injected into the light-emitting layer via the electron-injection layer and the holes injected into the light-emitting layer via the hole-transporting layer recombines with each other, and this recombination causes light in the light-emitting layer, and the light generated in the light-emitting layer travels outwards through the transparent electrode and the transparent substrate.
Such an organic electroluminescent element is characterized, for example, in that it is self-luminous and that the element shows relatively high-efficiency emission characteristics and permits emission of light in various color tones. Specifically, their use is highly expected as light emitters in display devices, such as flat panel displays or as light sources, for example, as backlights for liquid crystal displays and lighting and some of them are already commercialized.
Such an organic electroluminescent element has a trade-off between the luminance and the lifetime. Thus, intensive developments aimed at an organic electroluminescent element that retains its lifetime unaffected even when the luminance of light is increased to obtain a more brilliant image and or to obtain brighter light are in progress. Specifically, there have been proposed organic electroluminescent elements in which multiple light-emitting layers are interposed between positive and negative electrodes and the light-emitting layers are electrically connected (see, for example, Patent Documents 1 to 6).
FIG. 4 shows an example of the structure of such an organic electroluminescent element. A plurality of light-emitting layers 4a and 4b are formed between an electrode serving as a positive electrode 1 and an electrode serving as a negative electrode 2. The plurality of light-emitting layers 4a and 4b are stacked such that an intermediate layer 3 is interposed between the neighboring light-emitting layers 4a and 4b. This laminated structure is placed on a surface of a transparent substrate 5. The positive electrode 1 is formed as a light-transmitting electrode, while the negative electrode 2 is formed as a light-reflecting electrode. An electron-injection layer and a hole-transporting layer formed on both sides of the light-emitting layers 4a and 4b are not shown in FIG. 4.
In such a configuration, the intermediate layer 3 is interposed between the plurality of light-emitting layers 4a and 4b so as to electrically connecting them to each other. When voltage is applied between the positive electrode and the negative electrode, the plurality of light-emitting layers 4a and 4b emit light simultaneously as if they are connected in series with each other. Since light emitted from the organic electroluminescent element is a total of light emitted from respective light-emitting layers 4a and 4b in this case. Hence, when a constant current is supplied, luminance is higher in this organic electroluminescent element than that in a conventional organic electroluminescent element. Thus, the problem of the trade-off between the luminance and the lifetime can be solved.
Examples of known common configurations of the intermediate layer 3 include, for example, (1) BCP:Cs/V2O5, (2) BCP:Cs/NPD:V2O5, (3) in-situ reaction products of a Li complex and Al, (4) Alq:Li/ITO/hole-transporting materials, (5) mixed metal-organic layers, (6) oxides of alkali metals and alkali-earth metals, (7) laminates of N-doped layer/metal oxide layer/P-doped layer and the like. In the formulae above, the character “:” means a mixture of two materials, while the character “/” means a laminate of the two former and latter compositions.